Electrical resistor



Get. 19, 1943. PQDQL$KY 2,332,255

ELECTRICAL RESISTOR Filed March 10, 1942 Patented a. 19, 1943 UNITED STATES PATENT, OFFICE I 2,332,255 ELECTRICAL nssls'ron Leon rodolsky, Pittsiicid, Mala, assignor to Sprague Specialties 00., North Adams, Mala, a corporation of Massachusetts Ap lication March 10, 1942, Serial No. 434,121

9 Claims. (Cl. 201-67) The present invention relates to electrical resistors and to an improved method of constructing resistors, particularly those of high wattage and of high ohmic resistance.

Such resistors are subjected in operation to high temperature rises, of 250 C. or more, which place severe requirements on the resistor, and must be met in a manner to insure long life and constancy in electrical characteristics of the resistor. For -xample, such high temperature rises cause important diiferentlal expansion 0! the resistor components resulting in severe stresses therein. Moreover, the high operating temperatures limit the choice of materials for the components, particularly in high-voltage applications.

To protect the resistor element, its insulating medium and its other components against humidity, oxidation and corrosion, it is desirable to hermetically seal the resistor, whereby, however, the above problems are further aggravated.

It is'an object ofthe invention to provide a hermetically sealed resistor meeting all of the above requirements, and which at the same time which:

Figure 1 is a partly sectionalized view of a wire-wound resistor in accordance with the invention.

Fig. 2 is a partly sectionalized view illustrating y a second embodiment of the invention.

the resistor of Fig. 2.

Referring to Fig. 1, the resistor shown comprises a cylindrical core i0 supporting a resistance element II which extends substantially to. the ends of the core. Core it consists of a nonhydroscopic, heat-resistant and electrical-insulating material such as porcelain, Isolantite or of other ceramic material, or of a heat-resistant electrical glass such as "Electrical Pyrex. The core I0 is preferably hollow as shown, but may also be solid throughout.

The resistor element Ii is shown to consist ofawound resistance-metal wire it, for example, nickel-chrome wire. The dimensions of the wire I! are determined in known manner by the Fig. 3 is an end view of one form of corefor desired resistance value and current carrying requirements of the resistor element. For resistors of comparatively low ohmic value the wire is wound in a single layer as shown, whereas for resistor of high ohmic value a multilayer type winding is preferred. Adjacent turns of the wire I! are insulated from each other by a suitable temperature-resistant insulating medium, such as a vitreous enamel or glass or other refractory insulating material, preferably formed as an adherent coating I; on the wire.

The resistance-element, instead of being wirewound, may be of other type, for example a sprayed or adhesiveiy bonded coating of carbon provided on the core It, or may consist of a solid matrix of a suitable resistance material, for example of a molded mixture of carbon and "Bakelite, requiring no supporting core.

Cast over each end of the core III are metal cups Il-M which by means of their flanges iB-IS make electrical connections with the respective ends of the element I i. Th cups ll-il are preferably of an alloy having a melting point of about 275 C. to 325 C. and may be an alloy of zinc with approximately 4% aluminum and 5% magnesium by weight. For reasons later to be pointed out, the flanges l5-l5 have a thickness in excess of the height of the element Ii.

The element II with its core I0 is enclosed within a cylindrical casing is consisting 0! a moistureand heat-resistant insulating material such as glazed porcelain, Isolantite or Electrical Pyrex. .Electrical Pyrex and particu larly if it is specially annealed, is exceptionally suitable because of its ruggedness, low temperature-coeilicient of expansion, and its resistance to mechanical and thermal shock.

As a feature of the invention, the casing i6 is sopositioned relative to the core It by the flanges lkli that a void or space 33 is formed between the casing and the resistance-element which affords unrestricted temperature expansionof the resistance element thereby obviating stresses in same during operation.v The space 33 preferably contains a gaseous medium to be referred to more fully'l'ater, and in addition may be filled with a suitable heat transfer medium such as quartz powder.

Preferably the casing i8 and the flanges l5li engage each other with a snug fit, which may be provided by the interposition of a heat-resistant gasket 30, for example of glass cloth.

For the moisture-proof hermetical sealing of the entire resistor, the ends of the casing it are closed by metal cap members l1 and It, for example ferrules of brass. bronze or the like, the flan 35 of which are formed with integral radiallyextending lips lQ-IS. Each of the ferrules i1 and i8 is Provided with a central aperture 3!, and the ferrule I! in addition has an offset aperture 32. The purpose of these apertures will later be pointed out.

The ends of the casing l5 are provided with coatings 2l-2l of lead, tin, silver, or of other suitable bonding medium by which the flanges 35-35 of the ferrules are secured to the casing by soldering, brazing or thelike as at 36. Coatings 2| may be applied, for example, by spraying or by chemical or electrochemical deposition, and preferably the ends of the casing are suitably roughened as by sand-blasting, to facilitate the adherence of the coating.

To prevent end-play of the core ill with its superimposed resistance element within the easing it, a flexible backing member, for example a spring washer 23, is interposed between each cast cap I! and the inside surface of its adjacent ferrule.

External electrical connection to the respective ends of resistor element II is made by wire conductors 24-24, which are embedded in the cast caps ,H-H and which pass through the aperture 3l-3l. Solder plugs 25-25 affix the respective wires to the ferrules il-ia and seal the apertures 3 i-3 I.

By closing the aperture 32 of the ferrule H a hermetically-sealed resistor assembly is obtained which is capable of satisfactory operation under normal conditions. I have found, however, that where severe operating conditions are encountered, the gaseous medium permeating the space 33 and other spaces within the resistor must not be permitted to develop a pressure signiflcantly greater than the pressure of the atmosphere surrounding the resistor. If this occurs the seal between the caps I1 and i8 and the casing i8 may be disrupted or at least weakened and the performance ofthe resistor correspondingly affected. It. istherefore' an important feature of the invention that the space 33 and other voids, for. example, the space within the core ID, are at least partially evacuated, so that at the highest resistor operating temperature the pressure within the casing is not significantly greater and is preferably less than that of the surrounding atmosphere.

For this purpose the resistor of Fig. 1 comprises. a temporary vent consisting of a metal tube 28 forming a path through the cast cap l4 connecting the space 33 with the exterior of the resistor through the aperture 32.

The space 33 is in turn interconnected with the space within the core III by one or more apertures 28 provided in the core. In some instances the vent-tube 26 may be dispensed with, in which case the path of the gas to be removed is through the porous gasket 30. Similarly, the resistor may be evacuated through the aperture 3| prior to the sealing thereof instead of through the aperture 32 as shown.

The resistor is evacuated preferably by operating the same at its rated or slightly higher than its rated power. This causes the enclosed gas to expand and flow through the tube 25. When equilibrium is reached and while the resistor is at the so induced temperature, the aperture 32 and tube 25 is sealed by a solder plug 21.

The resistor may also be evacuated to the required degree, by a vacuum pump applied to the aperture 32. In general, for a resistor which in operation assumes a temperature of approximately 275 C., and which is operated in a surrounding atmosphere at normal pressure of 760 mm, the pressure within the casing at room temperature should be approximately '76 mm.

In certain applications it is necessary to prevent entirely any oxidation whatever of the resistor element. To meet this requirement the resistor is filled with inert gas such as nitrogen. for example by first completely evacuating the resistor and thereafter filling the same with the inert gas to the required pressure.

The resistors of the invention are rugged and simple to manufacture and remain hermetically sealed under even the most severe operating conditions. Such resistors withstand without damage ten and even more cycles of the following treatment.

1. Operate resistor at ratedpower for six (6) hours.

2. Immediately immerse resistor in water at 0 C.

3. Operate resistor at rated power for two (2) hours.

4. Immediately immerse resistor into a saturated sodium chloride solution at a temperature of 100 C., and maintain resistor immersed for two (2) hours.

5. Immediately immerse resistor into a saturated sodium chloride solution at 0 C., and maintain immersed for two 2) hours.

6. Wash in clear water.

7. Immediately apply rated power to resistor for six (6) hours.

The resistor of the invention is characterized by other important advantages.

Because the resistor remains hermetically sealed under even the most rigorous conditions, corrosion of the resistance element is minimized to a negligible extent. This makes possible the use of resistance wire of even the smallest diameter for the winding without incurring any significant change in the life and service of the resistor. By means of the invention, compact resistors are obtained having a high ohmic value and being capableof withstanding highly corrosive atmospheres, such as salt spray.

Moreover, the invention has made possible for the first time resistors with high ohmic values and power ratings utilizing complex types of windings, for example, of the bank and progressive universal types, thereby obtaining the most efficient utilization of the winding space within the resistor.

Fig. 2 illustrates a second embodiment of the invention as applied to resistors of the multiplier type having high ohmic values and suitable for operation at high voltages, for example of the order of nine thousand (9000) volts.

The resistor shown comprises a cylindrical core 40 in the form of a series of interconnected spools having individual winding spaces defined by integral barriers 4| and within which a resistor element is contained. Core 40 consists of a suitable refractoryinsulating material such as porcelain, or Isolanitite or of electrical glass.

The resistor element is divided into a plurality of individual interconnected sections to minimize voltage stresses, each of said sections consisting of a multilayer type resistance wire winding 42 disposed within the space defined by two adjacently disposed barriers 4|. I

For the "passage of the wire interconnecting the windings, the barriers 4| are formed with a radial slot 43, such being shown in greater detail in Fig. 3.

aasaass The core 40 and windings 42 are enclosed within a moistureand heat-resistant insulating casing 44 consisting preferably of annealed "Electrical Pyrex" for the reasons previously pointed out in connection with the resistor of Fig. 1.

For the moisture-proof hermetlcal sealing of the resistor. the ends of the casing 44 are closed by ferrules 45 and 45 of brass, bronze or the like, the flanges 60 of which are each formed with an integral radially-extending lip 49. Each of the ferrules 45 and 45 is provided with a central aperture 41.

Each end of the casing -44 is provided with a coating 50 of lead, tin, silver or other suitable bonding medium by which the flanges 60-60 of the ferrules are secured to the casing by soldering, brazing or the like, for example as shown at Coatings 50-50 may be applied, for example, by spraying or by chemical or electrochemicaldeposition, and preferably the ends of the casing are suitably roughened, for example by sand-blasting, to facilitate the adherence of the coatings.

External electrical connection to the respective ends of the resistor windings is made by connecting wires which pass through the apertures 41-41 and are secured to the ferrules 45-45 respectively, by solder plugs 52-52, which plugs also seal the apertures.

To prevent end-play of the core 40 with its superimposed windings within the casing 44, a resilient backing member, for example a spring washer 53 is interposed between the inside surface of each ferrule and the respective end of the core 40.

The resistor is evacuated by means of a heat treatment or by means of a vacuum pump as previously described, whereby the enclosed gases are removed through one or both of the apertures 41 which are thereafter sealed. by the solder plugs 52-52.

In the following claims the terms normal temperature or "atmospheric temperature" and normal pressure or atmospheric pressure should be understood to refer to standard atmospheric conditions as established in engineering practice, 1. e. a temperature of approximately 25 C., and a pressure of approximately 760 mm. The term operating temperature of the resistor is to be understood to refer to the temperature assumed by the resistor when rated power is applied thereto, and the term "operating pressure" is to be understood to be the pressure within the resistor casing at the operating temperature of the resistor.

While I have described my invention by means of specific examples and in specific embodiments, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

1. An electrical resistor comprising a resistance element, an open-ended insulating casing spacedly surrounding said element and extending to the ends thereof, cap members closing each end of the casing, means hermetically sealing said cap members to the casing, and a gaseous filling within said casing, said sealing means being rupturable when subjected to a substantial gaseous pressure diiferential at the operating temperature thereof, and said gaseous filling at the operating temperature of the resistor having a pressure value approximately equal to the pressure value of the medium surrounding the resistor.

2. An electrical resistor comprising a resistance element, an open-ended insulating casing spacedly surrounding said element and extending to the ends thereof, a metallic layer forming an integral coating over each end of the casins. cap members closing each end of the casing,

. means hermetically sealing said cap members to and having its end portions formed with a roughened surface, metallic layers forming an integral coating over each of said roughened surfaces, cap members closing each end of the casing, means hermetically sealing said cap members to said coatings, and a gaseous filling within said casing, said sealing-means being rupturable when subjected to a substantial pressure differential at the operating temperature thereof and. said gaseous filling at the operating temperature of the resistor having a pressure value approximately equal to the pressure value of the medium surrounding the resistor.

4. An electrical resistor comprising a resistance element, an open-ended insulating casing spacedly surrounding said element and extending to the ends thereof, cap members closing each end of the casing, means hermetically sealing said cap members to said casing, and an inert gaseous filling within said casing, said sealing means being rupturable when subjected to a substantial gaseous pressure differential at the operating temperature thereof and said gaseous filling at the operating temperature of the resistor having a pressure value approximately equal to the pressure value of the medium surrounding the resistor. A

5. An electrical resistor comprising a resistance element, an open-ended insulating casing surrounding said element, means spacedly positioning said element in said casing, a porous heattransferring electrical insulating medium within the space, cap members closing each end of the casing, means hermetically sealing said cap members to said casing, and a gaseous filling permeat ing said heat-transfer medium, said sealing means being rupturable when subjected to a substantial gaseous pressure differential at the operating temperature thereof and said gaseous filling at the operating temperature of the resistor hays ing a pressure value approximately equal to the pressure value of the medium surrounding the. resistor.

6. An electrical resistor comprising an insulating core comprising spaced radially-extending barrier portions, individual resistance wire windings on the core each interposed between two adjacently disposed barriers, an insulating casing consisting of a glass having a'low temperature coefiicient of linear expansion surrounding the core and extending to the ends thereof, a roughened surface on each end of the casing, metallic bonding layers forming integral coatings over the roughened surfaces, cap members closing each end of the casing, means hermetically sealing said cap members to said coatings, and a gaseous filling within the casing and permeating the spaces defined by said barriers, said sealing means being rupturable when subjected'to a substantial pressure differential at. the operating temperature thereof and said gaseous filling at the operating temperature of-the resistor having a pressure value approximately equal to the pressure value of the medium surrounding the resistor.

7. In the manufacture of an electrical resistor, the steps comprising disposing about a resistance element an open-ended casing of a heat-resistant gas-impervious insulating materiaL heating said element to a temperature approximating the operating temperature of the element and for an interval sufllclent to reach heat equilibrium in the resistor and thereafter without substantially cooling the resistor hermetically sealing the I casing.

8. An improved resistance element, capable of operating at high temperatures and of withstantial pressure diflerential at the operating temperature thereof but permitting sudden and marked cooling of said resistor from its operating temperature without rupturing the casing, the pressure difierential between the inside and outside of said resistor at its operating temperature being insuincient to rupture the atoresaidhermetic seal.

9. An improved resistance element capable of operating at high temperatures and of withstanding the thermal shock produced by suddenly and markedly reducing the temperature of the casing thereofrrom said operating temperature, which comprises in combination a core upon which is coiled an electrical resistance wire coated with a temperature-resistant insulating medium and so positioned that it is capable oi? substantially unrestricted temperature expansion, an open-ended cylindrical insulating casing of moisture and heat-resistant specially annealed glass spacedly surrounding said resistance coil, a metallic layer bonded on the outside surface of said casing at either end and extending beyond the hereinafter described cap members, cap members closing said casing and hermetically sealed to the metallic bonding layer thereon, a

'gaseous filling within said casing, said sealing means being rupturable when subjected to a substantial pressure diilferential at the operating temperature thereof but permitting sudden and marked cooling or the resistor from its operating temperature without rupturing the casing, the pressure diflerential between the inside and outside of said resistor at its operating temperature being insuiflcient to rupture the aforesaid hermetic seal.

LEON PODOLSKY. 

